Such an actuator is described in the specification of U.S. Pat. No. 4 671 400, and enables the clutch not only to be motorised, but also to operate utterly reliably, regardless of the state of wear of the friction liners of the clutch. At the same time it affords a very simple construction with minimum movement of the various maneuvering means of the clutch. The coupling means are part of a wear compensating arrangement to which the above-mentioned two-part coupling means are especially applicable. The two parts of the coupling means are normally snap-fitted to each other. This arrangement does give satisfaction, but it does not enable continuous wear compensation to be obtained.
When the maneuvering means or actuators are of the fluid operated type and include a main piston and a main cylinder, together defining a control chamber, the arrangement mentioned above is difficult to fit within the actuator itself.
In addition, the displacement of the linkage and therefore of the diaphragm, is then a function of the hydraulic or pneumatic fluid pressure prevailing in the control chamber. This is disadvantageous in the context of a clutch having a diaphragm with a characteristic curve of "saddlebacked" shape, i.e. showing a maximum of declutching force followed by a decrease. Thus there may be more than one clutch position corresponding to a given fluid pressure, so that the clutch is not actuated in a bi-uniform manner. Bi-uniform operation of the clutch is however desirable--that is to say, for a given fluid control pressure, there will be only one definite clutch position.